Force nodes for SSL tests to start in TCP mode

Currently they are started in unix socket mode in ost cases, and then
converted to run in TCP mode. This can result in port collisions, and
there is no virtue in startng in unix socket mode, so start as we will
be going on.

Discussion: https://postgr.es/m/d6ee8761-39d1-0033-1afb-d5a57ee056f2@gmail.com

Backpatch to all live branches (12 and up).

src/test/ssl/t/SSLServer.pm

@@ -42,6 +42,12 @@ our @EXPORT = qw(
   switch_server_cert
 );
 
+# Force SSL tests nodes to begin in TCP mode. They won't work in Unix Socket
+# mode and this way they will find a port to run on in a more robust way.
+# Use an INIT block so it runs after the BEGIN block in TestLib.pm.
+
+INIT { $TestLib::use_unix_sockets = 0; }
+
 # Copy a set of files, taking into account wildcards
 sub copy_files
 {